Method of manufacturing small current selenium rectifiers



Aug. 1 ,1949.

WITNESSES:

Method of Manufacturing Selenium Rectifiers l0 mil. Backing Plate Coatedwith Selenium Selenium Surface Coated with Cd 8 Counter ElectrodeSublimed on O .l mil. Thick Plate HeatTreated on 207C Small Discs.Punched out of Plate l Ni! ENTOR Earl D. Wilson.

ATTORNE Patented Aug. 16, 1949 METHOD OF MANUFACTURING SMALL OUR- RENTSELENIUM RECTIFIERS Earl D. Wilson, Wilkinsburg, Pa., assignor toWestinghouse Electric Corporation, East Pittsburgh, Pm, a corporation ofPennsylvania Appllcatlen May 10, 1946, Serial No. 868,755

3 Claims. -1

My invention relates to selenium rectifiers and, in particular, relatesto a method or process for manufacturing such rectifier units havingelltremely small current ratings, such as of the order of twomilliamperes.

One object of my invention is to provide an improved method ofmanufacturing selenium rectifier elements in large quantities where thesize of the individual unit rectifiers is of the order of one-eighth ofan inch in diameter.

Another object of my invention is to provide an effective and economicalmethod of manufacturing selenium rectifier units of extremely smalldiameter by punching them from selenium rectifiers having the form ofsheets of much larger longitudinal dimensions.

Still another object of my invention is to provide a process by whichselenium rectifiers may be punched from larger rectifier units havingthe form of thin sheets of considerable surface area without the dangerof leaving metallic burrs short-circuiting the units after the finalpunching.

Other objects of my invention will become appari "it upon reading thefollowing description of preferred processes for carrying out theprinciples thereof. The attached drawing schematically illustrates suchprocesses.

For certain purposes, such as, for example, producing voltage sourcescapable of furnishing extremely small currents, 'of the order ofmilliamperes, at voltages of the order of many thousands, it isdesirable to make up selenium rectifiers comprising unit rectifier disksof the order of a small fraction of an inch in diameter, electricallyconnected in series with each other, in suflicient numbers to yielddirect-current output voltages of the order of from 10,000 to 15,000.Since the rectifier elements can withstand something of the order of 30volts per unit disk, this means an aggregation of several hundredsmall-diameter disks. For example, to produce output currents of 2milliamperes, I have found that disks about an eighth of an inch indiameter may be employed. The production of extremely large numbers ofsuch small-diameter disks offers a problem of considerable technicaldifficulty if it is to be done economically.

The process which I have found to solve the foregoing problem isscheduled in the drawing in a way believed to be self -explanatory andconsists in first forming selenium rectifiers in sheets of substantialdiameter, and thereafter punching disks of a diameter of about an eighthof an inch from the sheet rectifier thus formed. In

accordance with this process, I may, for example, take washers stampedfrom nickel sheet 10' mils thick, having one surface sand-blasted. Sixpairs of such disks are mounted back to back on a spindle with aquarter-inch spacing between the pairs. They are then coated withselenium which contains 0.01% of bromine by submerging the spindle in amolten bath of that material maintained at 350 C. gheis'pindles arepreferably rotated slowly while in the bath, and upon withdrawal/the;efi om,' are rotated at a sufficient speed so that,cntrifugal forcecarries of! a large part of the molten selenium, leaving a layer havinga thickness of about 2 mils on the nickel. After cooling, the disks aredisassembled and the surface of the selenium layer is coated with a fullgreen film of cadmium sulphide by evaporating the latter in vacuo andpermitting it to condense on the selenium surface. A 1 mil layer of analloy comprising cadmium and 20% tin is sprayed onto the disks, asuitable mask being provided for protecting the edges of the disks fromsuch coating. The disks are then heat treated at C. for 80 minutes andthereafter submitted to a further heat treatment at 207 C. for minutes.Thereafter the disks are con nected in an electric circuit whichimpresses a direct-current voltage in their normal non-conductivedirection, such voltage varying from 30 volts at the beginning of theperiod and rising gradually to '70 volts at the end of two hours. Duringthis so-called electrical formation, the disks are cooled by air blasts.

The sheet rectifiers thus produced are then punched up into small unitsabout an eighth of an inch in diameter. It is found that if this is donewith an ordinary punch and die, the sharp edges of the punch damage theportion of the barrier layer of the rectifier around the periphery ofthe unit and, furthermore, is likely to create a short-circuit betweenthe counter electrode of cadmium alloy and the metal base of nickel bycausing metal to flow plastically to produce bridges between the twounder action of the pressure. I have, however, found that the damage tothe barrier layer may be avoided by inserting a sheet of stiff paperbetween the cadmium alloy side of the sheet and the die, thusdistributing the punching forces sufliciently to keep the pressuresbelow the critical value at which such damage results. The diilicultyfrom metal bridging between the base plate and counter electrode iscorrected by dipping the finished disks in an etching solutioncomprising 10% nitric acid in distilled water at room temperature. Thesmall punched disks may be loaded into a basket of nickel wire screen;dipped into the above-mentioned solu tion for 5 minutes; the basket andits load being thereafter rinsed thoroughly with tap water, then withdistilled water and thereafter dried in a strong blast of air, followedby baking :in an oven at 120 C. for 30 minutes. Any paper adhering tothe disks as a result of the punching process is automatically loosenedand washed away in this procedure.

I have discovered an alternative method of avoiding the short-circuitingcaused by the metal bridges in the punching operation. Instead ofcoating the selenium surface with the above-mentioned alloy of cadmiumand tin to a thickness of about a mil, as above described, the alloy maybe applied to the selenium surface of a thickness of only aboutone-tenth of a mil, preferably by evaporation and sublimation. with thecounter electrode put on in this fashion,

- the short-circuiting is found not to occur during the punchingoperation.

Rectifier elements made in accordance with my process may convenientlybe assembled into high-voltage low-current rectifiers by providing fibertubes having central holes slightly larger in diameter than the disksand filling the desired length of these tubes with disks made accordingto the method above-described. End connections of the tubes may comprisebrass ferrules, and a firm connection between the successive disks maybe maintained by providing a proper spring to exert pressure between oneferrule and the top unit of the stack in the tube.

I claim as my invention: a

1. The process of producing selenium rectifle which comprises firstproducing selenium rectifiers having counter-electrodes in the form ofsheets having extended area, covering one face of the sheet rectifierwith stiff paper, thereafter punching small rectlfiers out 0! suchsheets, and

4 thereafter immersing the punched units in a chemical solution having asolvent action. on the counter-electrode material.

2. The method of producing selenium rectifiers of small surface areawhich comprises coating a backing plate having a thickness of the orderof 10 mils with a layer of selenium having a thickness of the order of 2mils, coating the surface of the selenium with a film of cadmiumsulphide, depositing on the surface of the sulphide a counter electrodehaving a. thickness of the order of 0.1 mil, and thereafter subdividingsaid sheet into units of the desired surface area by a punchpressingoperation.

3. The method of producing selenium rectifiers of small surface areawhich comprises coating at least one face of a 10 mil nickel sheet witha layer having a thickness of the order of 2 mils by punching operation.

EARL D. WILSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,229,807 Hoppe Jan. 28, 19412,244,664 Addink June 10, 1941 2,417,839 Richards et al Mar. 25, 1947

